This invention relates to methods of operating fluidized bed systems combining a dense fluidized bed and an entrained fluidized bed wherein the beds contain particle components having long-term chemical and physical stability in the system, one particle component being retained in the dense fluidized bed while the other is entrained and recirculated through a portion of the dense bed to increase residence time and promote more complete reaction of the reactant feed particles therein. In particular, the invention relates to the operation of such a multi-solid system wherein the dense bed is operated with two distinct temperature zones for optimum control of a fuel combustion or gasification and capture of released sulfur by an added sorbent. With slight modification, the method is utilized for both combustion and gasification of high-sulfur coal.
Operation of a uniform temperature, multi-solid fluidized bed system is taught and described in copending U.S. Patent application Ser. No. 803,831, filed June 6, 1977, now U.S. Pat. No. 4,084,545 which is assigned to the assignee of this application and which is incorporated herein by reference.
Operation of fluidized bed systems at more than one constant temperature is taught, in general, by prior workers. U.S. Pat. Nos. 3,597,327 and 3,847,563 are two which suggest two-temperature operation. The former patent discloses a method for pyrolyzing coal in a fluidized bed unit wherein the unit is separated into a first region and a second region. The coal is introduced into the bottom of the first region and is pyrolyzed therein. The region is operated with two zones containing different size particles (coarse coke and fine dolomite, for example) but both components are in dense fluidized condition (because of a difference in the superficial gas velocity) and are contiguous rather than coextensive as in the present invention. The lower region (2 zones) is operated at a temperature of about 1400.degree. F. which is chosen for efficient coal carbonization. A second region operating at a higher temperature, about 1740.degree. F., may receive treated material from the first region.
U.S. Pat. No. 3,847,563 discloses a two-stage fluid bed gasifier wherein coal is devolatilized in one bed at 1300.degree.-1700.degree. F. and the char is cycled to a second fluid bed gasifier which operates at about 2100.degree. F. in the bottom and about 1800.degree.-2000.degree. F. at the top. Some of the char is combusted in the bottom and the resulting ash agglomerates and settles out. Remaining char is further gasified in the top of the bed as hot fluidizing air and steam pass out the top of the gasifier to be recycled to the devolatilizing bed. Sulfur is captured in the lower temperature devolatilization bed by lime and is removed therefrom as CaS.
Compared with previous fluidized bed combustion and gasification systems, the present invention allows reductions in unit size and heat transfer surface requirements because of higher superficial velocities which are obtainable, increases in combustion and gasification efficiencies and increased utilization of sorbent due to increased residence times of fuel and sorbent and precalcining of sorbent in the combustor.
Compared with the gasification in U.S. Pat. No. 3,847,563, the present gasification method takes place in one vessel and exposes the sorbent only to hot gases. In the prior patent two vessels are necessary. Sorbent is exposed to char in the devolatilization bed which then requires a separation if recycle is desired, and in the gasifier bed which causes agglomeration of sulfide with the ash resulting in an environmentally undesirable waste.